Seamless pipes produced by hot working such as the hot extrusion tube-making process and the Mannesmann tube-making process as line pipes are often used in pipe lines for transporting oil and natural gas. In installation of pipe lines, the seamless pipes are connected by successively joining their end faces to each other by welding. For this welding, the seamless pipes for use in line pipes are required to be excellent in dimensional accuracy of their pipe ends, particularly dimensional accuracy of their inner diameters. The seamless pipes produced in the same manner are often used as oil well pipes for oil wells and gas wells (collectively referred to as “oil wells”, hereinafter), and also required to have excellent dimensional accuracy of their pipe ends.
Recently, there has been a trend to require more enhanced dimensional accuracy in inner diameters of pipe ends of seamless pipes, and acceptable dimensional tolerance thereof has been increasingly tighter. Consequently, in seamless pipes for use in line pipes and oil well pipes, inner diameters of their pipe ends are required to be corrected. This pipe end correction is carried out by forcing a plug into a pipe end portion of a seamless pipe for inner diameter correction so as to expand the pipe end portion.
Prior art techniques pertinent to the pipe end correction of seamless pipes are as follows.
Patent Literature 1 discloses a lubricant for cold working made of alkali soap, which can also be applied to pipe expansion (pipe end correction) at a pipe end portion by use of a plug. This Patent Literature also discloses a technique that applies an alkali soap solution or a hydrous alkali soap paste to a surface to be processed for a seamless pipe, or a surface of a tool (an inner surface of the pipe end portion or the surface of the plug in the case of pipe end correction) so as to form a solid alkali soap film thereon, and thereafter carries out cold working (pipe end correction). In the technique disclosed in this Patent Literature, in the cold working of the seamless pipe, the lubrication film of the solid alkali soap is formed on the surface to be processed so as to reduce load during the cold working, and after the cold working, the water-soluble lubrication film can be easily removed by cleansing the processed surface with water or hot water.
Patent Literature 2 discloses a plug including a two-stepped tapered portion whose diameter gradually increases from the nose of plug, and a constant diameter portion having a constant diameter that back-to-back extends from the rear end of the tapered portion, in which a dimensional relation between the diameter of the constant diameter portion, and a taper angle and an axial length of each tapered portion is optimized. The technique disclosed in this Patent Literature uses the plug having a shape optimized in this manner in the pipe end correction so as to suppress a phenomenon that the inner diameter of the pipe end portion of the seamless pipe becomes excessively larger than the diameter of the constant diameter portion of the plug (overshoot), thereby enhancing the dimensional accuracy of the pipe end inner diameter.
Meanwhile, since pipe lines are exposed to corrosive gases such as carbon dioxide gas and hydrogen sulfide, seamless pipes for use in line pipes are required to have corrosion resistance and stress corrosion cracking resistance, and further required to have excellent properties such as weldability, toughness, and strength. The same properties are required in seamless pipes for use in oil wells. Martensitic stainless steels such as 13% Cr steels (13% Cr-0.2% C) specified by API (American Petroleum Institute) are often used in seamless pipes for use in line pipes. Recently, in order to further enhance corrosion resistance, improved 13% Cr steels containing extremely small amount of C, and containing Ni instead of C have been put into practical use. Austenite-ferrite based duplex stainless steels containing large amount of Cr such as 22% Cr steels and 25% Cr steels are often used in seamless pipes for use in oil wells.
In production of a seamless pipe made of a martensitic stainless steel represented by an improved 13% Cr steel, heat treatment is carried out after hot tube-making. It is also necessary to can out subsequent treatments such as shotblasting and acid pickling. Oxide scale generated during the hot tube-making and/or the heat treatment are accumulated in a layered manner on the inner and outer surfaces of the seamless pipe after the heat treatment, and such accumulated oxide scale hinders corrosion resistance required in the seamless pipe, and for this reason, the above treatments are carried out to remove the oxide scale.
Seamless pipes made of improved 13% Cr steels are also required to have tight dimensional tolerance in inner diameters of pipe ends, and thus the pipe end correction becomes essential. In the production of a seamless pipe of a martensitic stainless steel, the pipe end correction is carried out after removal of the oxide scale (see Patent Literature 3, for example).
In the case of production of a seamless pipe of a duplex stainless steel, the same treatments are applied as those in the production of the seamless pipe of a martensitic stainless steel, and the pipe end correction is carry out as well.